I took a look in the LXX using the rather imperfect data in the
UPenn LXX. I found a host of first person singular subjunctives
(over 500) so I did a proximity search on *DEURO + first
person singular subjunctive* which resulted with four
matches: Gen. 37:13, Ex. 3:10, Num. 23:27, Sam. 16:1.

Gen 37:13 DEURO APOSTEILW SE PROS AUTOUS

EX 3:10 KAI NUN DEURO APOSTEILW SE PROS PHARAW

NUM 23:27 DEURO PARALABW SE EIS TOPON ALLON

1SAM 16:1 DEURO APOSTEILW SE PROS IESSAI

If you take a look at these four examples in their context I
think you will see that they all translate nicely as declarative
futures. The imperatival tone is supplied by DEURO, but the
following subjunctive can be read as simply a statement about
what the speaker intends to do in the immediate future.

I am still wondering about subjunctives and future time. Is it
possible that we don't have a morphological future in the
subjunctive simply because it would be redundant
information? I know that subjunctives can be used in a
historical narrative. But isn't the subjunctive always future
relative to the person or persons who are caught up in the
action specified in the verb?